1. Field of the Invention
The present invention relates to compositions comprising an epoxy resin; a reagent selected from the group of cationic compounds or compounds which are capable to form cationic compounds or mixtures of those; at least one compound with two aromatic rings and at least one central cyclic oxygen bridged ring and the use of such compositions to reduce shrinkage.
2. Brief Description of the State of the Technology
Curable compositions containing epoxide and cationic initiator (Bronsted acid, Lewis acid, and there derivatives involving various latent initiators) are widely used for adhesives, sealants, and coatings. Additionally these compositions are regarded as the most promising ones for electrochip bonding and underfills.
However, these compositions suffer from their volume shrinkage during curing reactions. The volume shrinkage during the curing reaction causes the formation of voids and micro-cracks in the cured material, and thus results in lowering the mechanical strength of the material. Especially in such semiconductor package and assembly applications, shrinkage causes contact failure between the adhesive and the substrate, thus leading to adhesion failure. More importantly, however, moisture can reach the surface of the substrate through voids and cracks, damaging the semiconductor chip and other components.
The problem of volume shrinkage has been conventionally solved by addition of inorganic fillers. However, addition of fillers to curable composition causes serious increase in its viscosity to make it less fluid. Moreover, the addition of fillers affects properties of the cured material (decrease in mechanical strength) and consequently has influence on the adhesion strength.
To solve the problem of volume shrinkage, it is opportune to add shrinkage suppressive monomers as co-monomers, which can undergo some reactions under the conditions for the curing reactions of epoxide. Such solutions are well known in the state of the art. Several compounds having cyclic ether structures have been claimed as shrinkage suppressive monomers in epoxy-cationic curing reactions. For example spiroketal compounds (SK) have been disclosed in JP2002128785 A2.

In a further publication Takata, T. and Endo, T. reveal spiroorthoester (SOE), and bicyclorothoester (BOE) as shrinkage suppressive monomers (in Polym. Prepr. Jpn., 1988, 37, 241). These monomers are designed so that they undergo cascading double ring opening reactions to create large free volume, compensating shrinkage of epoxy monomers upon curing reactions. However, the shrinkage suppressive effects by these compounds are still not sufficient. Some of the monomers require exhaustive multi-step syntheses with low yields, leading to high costs in utilization of these monomers. SOE is prepared by acid-catalyzed addition reaction of epoxide and lactone and the resulting crude mixture is used as a crude SOE, reducing cost for the monomer synthesis (JP59149924 A2 by Toshiba Corp., Japan). However, this method gives crude SOE contaminated with the acid catalyst, which seriously damage pot life of epoxy formulations containing the crude SOE.

Besides the above mentioned cyclic ethers also lactones have been disclosed as shrinkage suppressive monomers. In WO2003076544 A1 and JP59199712 A2 aliphatic lactones are mentioned as shrinkage suppressive monomers. Also Spirobislactone (SBL) is known as a shrinkage suppressive monomer (J. Polym. Sci., Part A: Polym. Chem. 2005, 43(15), 3421-3432).
Nevertheless, the shrinkage suppression effects of those monomers were not satisfying.

Despite the mentioned state of the art there remains a need for effective shrinkage suppression in cationic-cured epoxy compositions.